Woot6600GT, im probably one of the most experieced people here with OC FAILURES. I have corrupt windows tons of times. OC can seriously screw your computer up!! The most common negitive effect from OC you will hear, is burning your computer up.
But this is the least experieced!
What is the most common, is corrupting windows. Basically, from OC windows gets trashed. The ending result is you cannot use windows any longer. Everything that was on certain partitions, cannot be accessed. So everything is lost, all your video games, music, programs, ect. The only way to fix this, is completely rebooting your computer. So you install windows all over, and start fresh. Sometimes you can replace a file, or fix the problem using the windows boot cd. But you have know how. For normal users, its to technicial.
I highly recoomend learning as much as possible before you OC. Also read that link Imation gave. Intil you completely understand it. You will also need some type of cooling system for your computer, if you want to OC. Along with a temperature monitor.
Finished With TF.